U.S. patent number 9,052,380 [Application Number 13/475,169] was granted by the patent office on 2015-06-09 for system for doppler positioning of seismic sensors and method.
This patent grant is currently assigned to Seabed Geosolutions AG. The grantee listed for this patent is Thomas Bianchi, Olivier Winter. Invention is credited to Thomas Bianchi, Olivier Winter.
United States Patent |
9,052,380 |
Winter , et al. |
June 9, 2015 |
System for doppler positioning of seismic sensors and method
Abstract
Method and system for determining positions of underwater
sensors. The method includes sending a Doppler variant signal from
a moving source; recording the signal with the at least one seismic
sensor; evaluating a frequency drift of the recorded signal; and
determining a position of the at least one seismic sensor based on
the evaluated frequency drift and a source movement relative to the
at least one sensor.
Inventors: |
Winter; Olivier (Massy,
FR), Bianchi; Thomas (Massy, FR) |
Applicant: |
Name |
City |
State |
Country |
Type |
Winter; Olivier
Bianchi; Thomas |
Massy
Massy |
N/A
N/A |
FR
FR |
|
|
Assignee: |
Seabed Geosolutions AG
(Laksevag, NO)
|
Family
ID: |
46084958 |
Appl.
No.: |
13/475,169 |
Filed: |
May 18, 2012 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20120300582 A1 |
Nov 29, 2012 |
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Foreign Application Priority Data
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May 26, 2011 [FR] |
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11 54609 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01S
5/18 (20130101); G01S 11/14 (20130101); G01V
1/3835 (20130101); G01S 3/8022 (20130101); G01S
13/505 (20130101) |
Current International
Class: |
G01S
5/18 (20060101); G01S 13/50 (20060101); G01S
11/14 (20060101); G01S 3/802 (20060101); G01V
1/38 (20060101) |
Field of
Search: |
;367/19,16 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2 396 014 |
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Jun 2004 |
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GB |
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03/001233 |
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Jan 2003 |
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WO |
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Other References
Australian Patent Examination Report issued in related Patent
Application No. 2012202826 and issued May 27, 2013 (References
cited in this Examination Report previously cited in an IDS filed
May 18, 2012). cited by applicant .
European Search Report issued in related EP 2 527 875 A1 with date
of completion of search being Jun. 21, 2012 (All references cited
in the Search Report were previously cited in an IDS filed May 18,
2012). cited by applicant .
French Search Report issued in related FR Application No. 1154609
and issued on Jan. 26, 2012. cited by applicant.
|
Primary Examiner: Hellner; Mark
Claims
What is claimed is:
1. A method for determining a position of at least one seismic
sensor when deployed underwater, the method comprising: sending a
first mono-frequency signal from a moving source; recording the
first signal with the at least one seismic sensor; evaluating a
frequency drift of the recorded first signal; determining a
position of the at least one seismic sensor based on the evaluated
frequency drift and a source movement relative to the at least one
seismic sensor; repetitively sending the first signal from the
source; recording locations of the source and associated times when
repetitively emitting the first signal; determining a modeled
frequency drift curve based on the first signal and its
corresponding times, radial velocities of the source, and medium
parameters; and determining a measured frequency drift curve from
the recorded pressures, particle velocities or particle
accelerations and their corresponding times.
2. The method of claim 1, wherein the first signal is a Doppler
variant signal which has a band-limited spectrum.
3. The method of claim 1, further comprising: sending a second
mono-frequency signal having a different frequency from the first
signal; and determining the position of the at least one seismic
sensor based on both the first and second signals.
4. The method of claim 1, further comprising: determining the
position of the at least one sensor by best fitting the modeled
frequency drift curve with the measured frequency drift.
5. The method of claim 4, wherein the measured frequency drift
curve is estimated by comparing recordings of a sensor moving along
with the source with recordings of the at least one seismic
sensor.
6. The method of claim 5, wherein the measured frequency curve at
the at least one seismic sensors is fitted to a measured frequency
curve measured at an at least one seismic sensor of a known
position.
7. The method of claim 6, further comprising: determining the
position of the at least one sensor relative to the at least one
sensor of known position by fitting respective measured frequency
curves.
8. The method of claim 1, further comprising: determining
uncertainties of the position of the at least one sensor.
9. The method of claim 1, wherein the first signal is a noise
produced by an engine of a boat that carries the source.
10. The method of claim 1, further comprising: determining the
drift frequency at the at least one sensor or at a central
processing device or at both the at least one sensor and the
central processing device.
11. A method for determining a position of at least one seismic
sensor when deployed underwater, the method comprising: emitting a
Doppler variant signal from a moving source; recording pressures,
particle velocities or particle accelerations and associated times
with the at least one seismic sensor fixedly attached to the bottom
of the ocean, wherein the pressures, particle velocities or
particle accelerations contain the emitted Doppler variant signal
modulated by the Doppler effect; measuring a frequency drift from
the recorded pressures, particle velocities or particle
accelerations; and determining a position of the at least one
seismic sensor based on the measured frequency drift and a source
movement relative to the at least one seismic sensor, wherein the
Doppler variant signal is one of a sine wave, a triangular wave, a
rectangular wave or a combination thereof.
12. The method of claim 11, further comprising: determining a
modeled frequency drift curve based on the emitted frequency and
its corresponding times, radial velocities of the moving source,
and medium parameters; and determining a measured frequency drift
curve from the recorded pressures, particle velocities or particle
accelerations and their corresponding times.
13. The method of claim 11, further comprising: determining the
position of the at least one sensor by best fitting the modeled
frequency drift curve with the measured frequency drift.
14. The method of claim 12, wherein the measured frequency curve at
the at least one seismic sensors is fitted to a measured frequency
curve at an at least one seismic sensor of a known position.
15. The method of claim 14, further comprising: determining the
position of the at least one sensor relative to the at least one
sensor of known position by fitting respective measured frequency
curves.
16. The method of claim 11, wherein the frequency drift is
estimated by comparing recordings of a sensor moving along with the
source with the at least one seismic sensor.
17. A system for determining a position of at least one seismic
sensor when deployed underwater, the system comprising: a moving
source configured to send a Doppler variant signal having a first
frequency; the at least one sensor configured to record pressures,
particle velocities or particle accelerations and associated times,
wherein the pressures, particle velocities or particle
accelerations contain the Doppler variant signal modulated by
Doppler effect; and a control device configured to, receive data
from the source and from the at least one sensor to calculate a
frequency drift from the recorded pressures, particle velocities or
particle accelerations, and determine a position of the at least
one seismic sensor based on the calculated frequency drift and a
source movement relative to the at least one sensor, wherein the
Doppler variant signal is one of a sine wave, a triangular wave, a
rectangular wave or a combination thereof.
18. The system of claim 17, wherein the control device is further
configured to: determine a modeled frequency drift curve based on
the emitted frequency and its corresponding times, radial
velocities of the source, and medium parameters; and determine a
measured frequency drift curve from the recorded pressures,
particle velocities or particle accelerations and their
corresponding times.
19. The system of claim 17, wherein the control device is further
configured to: determine the position of the at least one sensor by
best fitting the modeled frequency drift curve with the measured
frequency drift.
Description
BACKGROUND
1. Technical Field
Embodiments of the subject matter disclosed herein generally relate
to methods and systems and, more particularly, to mechanisms and
techniques for determining positions of underwater objects.
2. Discussion of the Background
During the past years, the interest in developing new oil and gas
production fields has dramatically increased. However, the
availability of land-based production fields is limited. Thus, the
industry has now extended drilling to offshore locations, which
appear to hold a vast amount of fossil fuel. Offshore drilling is
an expensive process. Thus, those undertaking the offshore drilling
need to know where to drill in order to avoid a dry well.
Marine seismic data acquisition and processing generate a profile
(image) of the geophysical structure under the seafloor. While this
profile does not provide an accurate location for the oil and gas,
it suggests, to those trained in the field, the presence or absence
of oil and/or gas. Thus, providing a high resolution image of the
structures under the seafloor is an ongoing process that requires
the deployment of many seismic sensors and the recording of various
seismic waves.
One method for recording the seismic waves is now discussed with
regard to FIG. 1. This method is appropriate when a distance from
the surface of the water to the bottom of the water is large, for
example, larger than 200 m. During a seismic gathering process, a
vessel 10 drags an array of seismic detectors 11 provided on
streamers 12. The streamers may be disposed horizontally, i.e.,
lying at a constant depth relative to a surface 14 of the ocean.
The streamers may be disposed to have other spatial arrangements
than horizontally. The vessel 10 also drags a seismic source array
16 that is configured to generate a seismic wave 18. The seismic
wave 18 propagates downwards toward the seafloor 20 and penetrates
the seafloor until eventually a reflecting structure 22 (reflector)
reflects the seismic wave. The reflected seismic wave 24 propagates
upwardly until it is detected by a detector 11 on the streamer
12.
However, the reflected seismic wave 24 (primary) is not only
recorded by the various detectors 11 (the recorded signals are
called traces) but also may reflect from the water surface 14 as
the water surface acts as a mirror for the sound waves, e.g.,
reflectivity one. The waves reflected by the water surface are
called ghosts in the art and these waves are reflected back towards
the detector 11. The ghosts are also recorded by the detector 11
but with a reverse polarity and a time lag relative to the
primary.
As discussed above, the recorded traces may be used to determine
the structure of the sub-structure (i.e., earth structure below
surface 20) and to determine the position and presence of
reflectors 22. However, to be able to determine the position of
reflectors 22, an accurate position of the detectors 11 is
necessary.
Another method for recording seismic waves uses fixed sensors
placed on the bottom of the region to be investigated as shown in
FIG. 2. This method is appropriate for shallow waters, when the
distance from the surface of the water to the bottom of the water
is 200 m or less. FIG. 2 shows the bottom 30 of the water and a
reflector 32 in the subsurface. A first vessel 34 tows a seismic
source 36 with the seismic source 36 being provided below the
surface 38 of the water. Detectors 40 are provided on the bottom 30
of the water. The detectors 40 are connected via cables 42 to a
recording vessel 44. This technology is called ocean bottom cable
(OBC). Ocean Bottom Seismometers may also be used for recording
seismic waves. The Ocean Bottom Seismometer is a self contained
data-acquisition system which free falls to the ocean floor and
records seismic data generated by airguns and earthquakes. Similar
to the method shown in FIG. 1, the positions of the detectors 40
need to be known in order to determine the position of the
reflector 32.
For determining the positions of the sensors for OBC, the following
techniques are common in the industry: (1) using the drop or
placement coordinates of the detectors, and (2) deploying
high-frequency acoustic sensors attached to the detectors and
positioned independently of the seismic survey and determining the
positions of the detectors based on the high-frequency acoustic
sensors. The positions of the sensors may be inferred by using the
first seismic source arrivals.
Because drop positions in the first technique must be recorded to
assure that the actual detector locations are near the planned
locations, drop positions are the cheapest and easiest to
implement. In calm shallow water (such as an inland bay where the
detectors may be placed on or thrust into the muddy bottom), the
detector drop position can be close to the resting position.
However, in deeper water or in agitated surf zones, this is
unlikely due to waves, currents and drop trajectories.
The second technique, which is disclosed in U.S. Pat. No.
4,641,287, the entire disclosure of which is incorporated herein by
reference, uses acoustic transponders located on a seismic cable
that connect the sensors. FIG. 2 shows acoustic transponders 46
placed at various positions. The acoustic transponders are
interrogated by a dedicated source boat (not shown). The acoustic
pulse's frequency emitted by the dedicated source boat is in the 30
kHz to 100 kHz range, i.e., a high frequency range. Repeating the
interrogation at different known locations allows the operator of
the boat to triangulate and deduce the precise pinger position of
the sensors 40.
However, there are not as many acoustic transponders as seismic
sensors. Furthermore, the acoustic transponders are located on the
cable, in-between the seismic sensors. Thus, the positions of the
sensors are interpolated from acoustic pingers positions, which
give approximate results.
A system described in U.S. Pat. No. 6,005,828, the entire
disclosure of which is incorporated herein by reference, couples
the acoustic transponders with the seismic sensors, which improves
the localization of the sensors.
However, the existing technologies are not capable to exactly
determine the positions of the sensors and also require the
presence of acoustic transponders, which make the entire equipment
complex and prone to failures. Further, if less transponders than
sensors are used, the accuracy cannot be improved over a certain
threshold. If each sensor is provided with a transponder, the
complexity and the weight of the system increases. Accordingly, it
would be desirable to provide systems and methods that provide an
accurate positions of the sensors without the acoustic
transponders.
SUMMARY
According to one exemplary embodiment, there is a method for
determining a position of at least one seismic sensor when deployed
underwater. The method includes a step of sending a first
mono-frequency signal from a moving source; a step of recording the
first signal with the at least one seismic sensor; a step of
evaluating a frequency drift of the recorded first signal; and a
step of determining a position of the at least one seismic sensor
based on the evaluated frequency drift and a source movement
relative to the at least one seismic sensor.
According to still another exemplary embodiment, there is a method
for determining a position of at least one seismic sensor when
deployed underwater. The method includes a step of emitting a
Doppler variant signal from a moving source; a step of recording
pressures, particle velocities or particle accelerations and
associated times with the at least one seismic sensor fixedly
attached to the bottom of the ocean, wherein the pressures,
particle velocities or particle accelerations contain the emitted
Doppler variant signal modulated by the Doppler effect; a step of
measuring a frequency drift from the recorded pressures, particle
velocities or particle accelerations; and a step of determining a
position of the at least one seismic sensor based on the measured
frequency drift and a source movement relative to the at least one
seismic sensor. The Doppler variant signal is one of a sine wave, a
triangular wave, a rectangular wave or a combination thereof.
According to still another exemplary embodiment, there is a system
for determining a position of at least one seismic sensor when
deployed underwater. The system includes a moving source configured
to send a Doppler variant signal having a first frequency; the at
least one sensor configured to record pressures, particle
velocities or particle accelerations and associated times, wherein
the pressures, particle velocities or particle accelerations
contain the Doppler variant signal modulated by Doppler effect; and
a control device. The control device is configured to receive data
from the source and from the at least one sensor to calculate a
frequency drift from the recorded pressures, particle velocities or
particle accelerations, and to determine a position of the at least
one seismic sensor based on the calculated frequency drift and a
source movement relative to the at least one sensor. The Doppler
variant signal is one of a sine wave, a triangular wave, a
rectangular wave or a combination thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute
a part of the specification, illustrate one or more embodiments
and, together with the description, explain these embodiments. In
the drawings:
FIG. 1 is a schematic diagram of a conventional system for
detecting the positions of towed sensors;
FIG. 2 is a schematic diagram of a convention system for detecting
positions of ocean bottom sensors;
FIG. 3 is a schematic diagram of a system for detecting positions
of sensors according to an exemplary embodiment;
FIG. 4 is a schematic diagram of a system that uses Doppler variant
signals for determining sensors according to an exemplary
embodiment;
FIG. 5 is a schematic diagram showing a radial velocity on a
map;
FIG. 6 is a graph showing recorded frequencies as a vessel moves
around the sensors according to an exemplary embodiment;
FIG. 7 is a schematic diagram of a geometry of the vessel relative
to the sensors;
FIG. 8 is a graph illustrating frequencies versus time according to
an exemplary embodiment;
FIG. 9 is a flowchart illustrating a method for determining
positions of sensors according to an exemplary embodiment;
FIG. 10 is a flowchart illustrating another method for determining
positions of sensors according to an exemplary embodiment;
FIG. 11 is a flowchart illustrating still another method for
determining positions of sensors according to an exemplary
embodiment; and
FIG. 12 is a schematic diagram of a control device configured to
determine positions of sensors according to an exemplary
embodiment.
DETAILED DESCRIPTION
The following description of the exemplary embodiments refers to
the accompanying drawings. The same reference numbers in different
drawings identify the same or similar elements. The following
detailed description does not limit the invention. Instead, the
scope of the invention is defined by the appended claims. The
following embodiments are discussed, for simplicity, with regard to
the terminology and structure of a set of sensors being deployed on
the bottom of the ocean. However, the embodiments to be discussed
next are not limited to these sets, but may be applied to sensors
being towed by a vessel or other devices whose positions underwater
need to be accurately determined.
Reference throughout the specification to "one embodiment" or "an
embodiment" means that a particular feature, structure, or
characteristic described in connection with an embodiment is
included in at least one embodiment of the subject matter
disclosed. Thus, the appearance of the phrases "in one embodiment"
or "in an embodiment" in various places throughout the
specification is not necessarily referring to the same embodiment.
Further, the particular features, structures or characteristics may
be combined in any suitable manner in one or more embodiments.
The technologies in marine seismic surveys need accurate positions
of the sensors deployed underwater. One novel approach for
determining the positions of the sensors deployed underwater is
illustrated with reference to FIG. 3. FIG. 3 shows a system 100
that include plural sensors 102 provided on the bottom 142 of the
ocean. Although the term "ocean" is used herein, one skilled in the
art would understand that this term also refers to a lake, pond,
sea, or other body of water. No acoustic transducers are provided
along cables 104 connecting the sensors 102 or on the sensors 102
for determining their positions. Instead, the sensors 102
themselves are used for determining their position as generically
described now. A source 120 is towed by a vessel 130. A control
system 122, disposed either on the vessel 130 or on the source 120,
is configured to drive the source 120 with a frequency that can be
recorded by the sensors 102. Waves 126 having one or more
frequencies are emitted by the source 120. A Doppler shifted
frequency is recorded by the sensors 102, as will be discussed
later. The Doppler shifted frequency and/or a frequency drift are
used in the following for determining the positions of the sensors.
The Doppler shifted frequency is a combination (addition,
subtraction or ratio) of the emitted frequency and the frequency
drift. In other words, the frequency drift is the absolute
difference between or the ratio of the emitted frequency and the
Doppler shifted frequency. Based on the Doppler shifted frequency
and some characteristics of the medium (e.g., speed of sound in the
water between the source 120 and the sensor 102), accurate
positions of the sensors 102 may be determined. Details of these
processes are now discussed with regard to the figures.
According to an exemplary embodiment, FIG. 4 shows that the sensors
102 may communicate through data cables 106 with a master vessel
108. A control device (controller) 110 provided on the vessel 108
may supply the necessary computing power for determining the
positions of the sensors. Alternatively, in node acquisition, i.e.,
if the data cable 106 is not present, the readings from the sensors
104 may be collected on a recorder 112 attached to the sensor and
then later retrieved by an autonomous vehicle (not shown). The
retrieved data may be provided to the control device 110 or 122 for
processing.
The position determining system 100 includes the seismic 120 that
is controlled by the controller 122. The controller 122 may be
provided on the vessel 130 or directly on the source 120. The
source 120 may be an acoustic source. A frequency range to be used
by the acoustic source may be less than 1.2 kHz. In this regard, it
is noted that the conventional methods use a high frequency source,
having a frequency range between 30 kHz and 100 kHz. Thus, the
acoustic source of this novel embodiment has a frequency much lower
than the sources used by the existing technologies. This frequency
difference creates the possibility to record the frequencies of the
acoustic source directly with the sensors 102. As the frequency
spectrum of interest from a seismic point of view is likely to be
between 1 and 300 Hz, one skilled in the art would appreciate that
a frequency emitted by the conventional positioning methods cannot
be recorded by the sensors 102 as this frequency is too high for
these sensors. Typical recording systems uses in the seismic field
are designed to record frequencies not larger than 2 kHz. In one
application, the source can have a higher frequency, for example,
an acoustic frequency (1 kHz to 20 kHz).
Because the vessel 130 moves around the expected positions of the
sensors 102, the moving source 120 emits a Doppler variant signal
in the sensor's bandwidth. In one application, the vessel 130 moves
with a constant speed, around 5 knots per hour. A global
positioning system (GPS) unit 124 may be located next to the source
120 (e.g., on the source or on the vessel) for locating the exact
position of the source 120 when emitting the Doppler variant
signal. A Doppler variant signal is defined, for example, as a
band-limited signal. The opposite of a band-limited signal is a
broadband signal. The ideal broadband signal is a Dirac signal,
i.e., an impulse signal.
Examples of Doppler variant signals are a sine-wave, a
triangular-wave, a square-wave or a combination of these signals at
different frequencies. Those skilled in the art can imagine other
Doppler variant signals. For example, a combination of different
sinusoids may be used as long as their frequencies are sufficiently
spaced apart (this is a function of the boat speed) so that the
sensors 102 can distinguish them. Also, interrupted band-limited
signals may be used.
Returning to the source 120, a wave 126 emitted by the source
propagates in all directions under water. FIG. 4 shows only the
waves 126 of interest. For simplicity, in the following it is
assumed that the wave 126 emitted by the source 120 has a single
frequency F.sub.source. However, one skilled in the art would
easily be able to extend to following method to waves having
multiple frequencies.
As the wave 126 propagates from the vessel 130 to the sensor 102
through the medium (e.g., sea-water) and as the vessel 130 moves
with a certain radial velocity relative to the sensor 102, a
frequency recorded by the sensor is different from the frequency
emitted by the source. This deviation (shift) can be calculated
using formula:
##EQU00001## where F.sub.source is the frequency of the wave when
emitted at the source, F.sub.obs is the frequency of the wave when
recorded at sensor 102, v.sub.radial is the radial velocity of the
vessel 130 relative to the sensors 102 and v.sub.water is the speed
of sound in the water.
FIG. 5 is a map showing a top view (in the X and Z coordinates) of
the source 120, vessel 130 and sensors 102. FIG. 5 also shows a
sail line 150 of the vessel 130, its actual velocity 152, and its
radial velocity 154. The radial velocity 154 is the projection of
the actual velocity 152 along a line that connects the source 120
to a sensor 102. For example, the radial velocity may be
determined, in Cartesian coordinates, based on the relation
##EQU00002## where x, y and z represent the position of the
recording sensor 102, X.sub.1, Y.sub.1, and Z.sub.1 represent the
position of the source at t.sub.1 and X.sub.2, Y.sub.2 and Z.sub.2
represent the position of the source at t.sub.2. A time difference
between t.sub.1 and t.sub.2 is assumed to be small for the above
relation to be precise enough for the purpose of this exemplary
embodiment. The speed of sound in water may be measured or
determined and the F.sub.source is controlled by the controller
122.
Thus, the F.sub.obs may be mathematically determined (assuming that
the positions x and y of the sensors are known) and at the same
time the F.sub.obs is available from the recordings of the sensors
102. Mathematical algorithms may be used to solve this inversion
problem, i.e., vary the positions x, y of the sensors 102 until a
good match is obtained between F.sub.obs measured and F.sub.obs
calculated. In order to achieve these results, as already
discussed, the accurate positions of the source as it passes the
sensors and corresponding time stamps need to be known. In
addition, the recordings of the sensors need to be time stamped for
a good correlation with the source.
The shape of the recorded F.sub.obs at three sensors 102A to C is
shown in FIG. 6. The geometry of the three sensors 102A to C are
shown in FIG. 7 relative to the passing vessel 130. FIG. 6 shows
the emitted frequency F.sub.source in time and the time variation
(Doppler drift) of the recorded frequencies at the sensors as the
vessel 130 moves relative to the sensors. It is noted that when the
vessel 130 passes a line L connecting the three sensors, the
frequencies recorded by the sensors are equal to the emitted
frequency as v.sub.radial is zero.
According to an exemplary embodiment, FIG. 8 is a plot of frequency
versus time that shows a curve 800 that corresponds to the measured
frequency determined by a sensor 102, a curve 802 that shows the
modeled frequency with a priori sensor location and a curve 804
that shows the modeled frequency with wrong sensor location. The
sensor location is the location estimated by the operator of the
vessel when deploying the sensors, which might be different from
the actual location of the sensors. It is noted the close
correlation between the measured curve 800 and the modeled curve
802 with a prior sensor location.
According to another exemplary embodiment, a process for
determining the positions of the sensors is now discussed with
regard to FIG. 9. As this process is intended to be as complete as
possible, it is noted that not all the steps need to be performed
for determining the positions of the sensors. In other words, some
steps to be described next are optional. As shown in FIG. 9, in
step 900 the sensors 102 are deployed at the bottom of the ocean
and their dropping coordinates are recorded. This information
constitutes the a priori sensor location discussed above with
regard to FIG. 8. However, this information is not accurate as the
sensor can move from the desired position due to various factors,
e.g., ocean currents.
In step 902, the vessel 130 moves around the sensors following
various lines of sail while sending Doppler variant signals. The
acoustic source used to generate the Doppler variant signal may be
a source commercially available. In one application, multiple
sources are used for generating multiple frequencies. In another
application, the engine of the vessel 130 may be used as the
acoustic source as this engine generates acoustic waves. A
sufficient source-sensor angle aperture is preferred for
determining an accurate position. The angle aperture is defined,
with regard to FIG. 5, as the angle between the actual velocity 152
and the radial velocity 154. A sufficient angle aperture would be
at least 70 to 90.degree. wide. In one application, the source
emits a continuous wave and the sensors continuously record the
arriving waves. However, the process can work even if the source
does not continuously emit the wave and/or the sensors do not
continuously record the waves.
In step 904, the source coordinates over time are recorded, for
example, using industrial GPS systems such as Differential Global
Positioning Systems (DGPS) or Real Time Kinematic (RTK). In step
906, the sensors recordings are time stamped and recorded to relate
them to the source GPS positions. For the sensors, it is noted that
various types of sensors may be used for determining the drifted
frequency. For example, geophones (speeds), hydrophones (pressures)
or accelerometers (accelerations) may be used as sensors. The
sensors may have the capability to determine themselves the drifted
frequency or transmit the recorded data to a general controller for
determining the drifted frequency.
The seismic sensors should record enough time to achieve the needed
aperture. Continuous recording is preferred, but not necessary. The
more redundancy, the more accurate the computed positions of the
sensors. In step 908, using the recorded pressures or velocities or
accelerations (depending on the type of sensor), the received
frequencies are selected for given time windows, and the frequency
drift is calculated relative to the emitted frequency, as a
function of time. It is noted that working with band-limited
signals allows to spread the energy over time, which is not the
case with impulsive methods.
The frequency drift estimation can be performed in the recording
device (some modifications of the recording device may be needed),
or on the seismic trace in real-time, or on the seismic trace at
post-processing. The two last possibilities take into account both
autonomous underwater recording systems (nodes on which it is
impossible to access sensor recordings in real-time) and
conventional recording systems (i.e., a recorder provided on a
master vessel and attached via a cable to the sensors). For
example, with regard to FIG. 4, the data necessary for determining
the positions of the sensors may be processed in the controller 110
or controller 122 or may be distributed for processing in both of
these controllers.
An alternative to the frequency selection discussed above, is the
use of a sensor 126 (near-field) provided next to the source 122,
e.g., on the vessel 130 so that no Doppler shift is recorded for
this moving sensor 126. The frequency drift may be estimated by
comparing the recordings of the moving sensor 126 with the
recordings of the static sensors 102.
In step 910, the sensors' absolute or relative coordinates are
estimated via an inverse-problem approach, i.e., find the
coordinates that best explain the measured frequencies over time.
In this step one or more of the following information may be used:
approximate sensors locations, source coordinates as a function of
time, approximate sound velocity in the water, stream models,
and/or a constraint on a well-known sensor position. Optionally,
the method can attach weights to each of the frequencies measured
in step 908 according to measurement uncertainties estimation.
In an optional step, uncertainties analysis for all estimated
sensors positions is performed. Thus, the operator of the sensors
may be provided not only with the estimated positions of the
sensors but also with the uncertainties (accuracies) of those
positions.
From an equipment point of view, it is noted that the novel method
discussed above can be implemented for the exiting sensors 102
without any modifications as the existing sensors are capable of
detecting frequencies in the range of 0 to 2 kHz. On the source
side, commercially available sources may be used or even the
hardware for the acoustic transponder pinging boats may be used if
the emitting frequency is modified to be in the range of the
sensor. These sources may be modified to include an amplifier and a
precise waveform generator that has the capability to time-stamp
the emitted waves.
Various methods to be implemented for determining the positions of
the sensors are now described with regard to FIGS. 10 and 11. In an
exemplary embodiment illustrated in FIG. 10, there is a method for
determining a position of at least one seismic sensor when deployed
underwater. The method includes a step 1000 of sending a first
signal having a first frequency from a moving source; a step 1002
of recording the first signal with the at least one seismic sensor;
a step 1004 of evaluating a frequency drift of the recorded first
signal; and a step 1006 of determining a position of the at least
one seismic sensor based on the evaluated frequency drift.
According to another exemplary embodiment illustrated in FIG. 11,
there is a method for determining a position of at least one
seismic sensor when deployed underwater. The method includes a step
1100 of sending a Doppler variant signal having a first frequency
from a moving source; a step 1102 of recording pressures, particle
velocities or particle accelerations and associated times with the
at least one seismic sensor fixedly attached to the bottom of the
ocean, wherein the pressures, particle velocities or particle
accelerations are generated by the Doppler variant signal; a step
1104 of calculating a frequency drift of the recorded pressures,
particle velocities or particle accelerations; and a step 1106 of
determining a position of the at least one seismic sensor based on
the calculated frequency drift. As discussed above, these methods
may be applied to sensors that fixed to the bottom of the ocean or
to sensors that are towed by a master vessel.
Optionally, the method described above may include a step in which
the measured frequency curve at the at least one seismic sensors is
fitted to a measured frequency curve at an at least one seismic
sensor of a known position, and/or a step of determining the
position of the at least one sensor relative to the at least one
sensor of known position by fitting respective measured frequency
curves.
With regard to where the calculations of the sensors positions are
performed, it is noted that these calculations may take place in a
control device (e.g., a processor) that is configured to perform at
least some of the steps discussed with regard to FIG. 9. More
specifically, the control device may be one of the controllers 110,
122 or another controller or a combination of them. For example, in
one application, data related to the source is collected by the
controller 122 and data related to the sensors 102 is collected by
controller 110. As will be described shortly, a controller may
include not only a processor but also a storage device for storing
data and other components.
The data from controller 122 may be transferred to the controller
110 and then the entire processing may take place at controller
110. Alternatively, data from controller 110 may be transferred to
controller 122 and then the entire processing may take place at
controller 122. Still another possibility is to transfer data from
both controllers 110 and 122 to another controller (not shown, for
example, a processing trailer or a processing centre after the
completion of the acquisition campaign) that has more computing
resources and then perform the entire processing at this
controller. The communications between the controllers may take
place via internet, radio waves, microwaves, satellite or other
known means in the art. The connections between the controllers may
be wired or wireless.
An example of a representative control device or controller capable
of carrying out operations in accordance with the exemplary
embodiments discussed above is illustrated in FIG. 12. Hardware,
firmware, software or a combination thereof may be used to perform
the various steps and operations described herein. The control
device 1200 of FIG. 12 is an exemplary computing structure that may
be used in connection with such a system.
The exemplary control device 1200 suitable for performing the
activities described in the exemplary embodiments may include a
server 1201, which may correspond to any of controllers 110 or 122
shown in FIG. 4. Such a server 1201 may include a central processor
(CPU) 1202 coupled to a random access memory (RAM) 1204 and to a
read-only memory (ROM) 1206. The ROM 1206 may also be other types
of storage media to store programs, such as programmable ROM
(PROM), erasable PROM (EPROM), etc. The processor 1202 may
communicate with other internal and external components through
input/output (I/O) circuitry 1208 and bussing 1210, to provide
control signals and the like. The processor 1202 carries out a
variety of functions as is known in the art, as dictated by
software and/or firmware instructions.
The server 1201 may also include one or more data storage devices,
including hard and floppy disk drives 1212, CD-ROM drives 1214, and
other hardware capable of reading and/or storing information such
as DVD, etc. In one embodiment, software for carrying out the above
discussed steps may be stored and distributed on a CD-ROM 1216,
diskette 1218 or other form of media capable of portably storing
information. These storage media may be inserted into, and read by,
devices such as the CD-ROM drive 1214, the disk drive 1212, etc.
The server 1201 may be coupled to a display 1220, which may be any
type of known display or presentation screen, such as LCD displays,
plasma display, cathode ray tubes (CRT), etc. A user input
interface 1222 is provided, including one or more user interface
mechanisms such as a mouse, keyboard, microphone, touch pad, touch
screen, voice-recognition system, etc.
The server 1201 may be coupled to other computing devices, such as
the landline and/or wireless terminals and associated applications,
via a network. The server may be part of a larger network
configuration as in a global area network (GAN) such as the
Internet 1228, which allows ultimate connection to the various
landline and/or mobile client devices.
In the detailed description of the exemplary embodiments, numerous
specific details are set forth in order to provide a comprehensive
understanding of the claimed invention. However, one skilled in the
art would understand that various embodiments may be practiced
without such specific details.
As also will be appreciated by one skilled in the art, the
exemplary embodiments may be embodied in a wireless communication
device, a telecommunication network, as a method or in a computer
program product. Accordingly, the exemplary embodiments may take
the form of an entirely hardware embodiment or an embodiment
combining hardware and software aspects. Further, the exemplary
embodiments may take the form of a computer program product stored
on a computer-readable storage medium having computer-readable
instructions embodied in the medium. Any suitable computer readable
medium may be utilized including hard disks, CD-ROMs, digital
versatile disc (DVD), optical storage devices, or magnetic storage
devices such a floppy disk or magnetic tape. Other non-limiting
examples of computer readable media include flash-type memories or
other known memories.
The disclosed exemplary embodiments provide a system and a method
for determining the positions of various sensors underwater. It
should be understood that this description is not intended to limit
the invention. On the contrary, the exemplary embodiments are
intended to cover alternatives, modifications and equivalents,
which are included in the spirit and scope of the invention as
defined by the appended claims.
Although the features and elements of the present exemplary
embodiments are described in the embodiments in particular
combinations, each feature or element can be used alone without the
other features and elements of the embodiments or in various
combinations with or without other features and elements disclosed
herein.
This written description uses examples of the subject matter
disclosed to enable any person skilled in the art to practice the
same, including making and using any devices or systems and
performing any incorporated methods. The patentable scope of the
subject matter is defined by the claims, and may include other
examples that occur to those skilled in the art. Such other
examples are intended to be within the scope of the claims.
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